How Bad Did Mopar’s 340 beat Ford & Chevy? Shocking New Data!

Everyone knows Mopar factory horsepower ratings in the muscle car era were the most honest of them all; if the 440 Magnum was advertised with 375 horsepower, that’s what it made. And let’s not forget how the 426 Street Hemi was cleverly under-rated by the ad guys. Rather than tout its nearly 500 horsepower at 6,000 rpm (and give the safety watchdogs even more to chew on), Chrysler intentionally stopped counting at 5,000 rpm, where 425 horsepower was legitimately on tap.

But what about the 340, launched in 1968 specifically for lightweight A-Body installations, then spread throughout the B- and E-Body performance models in 1970-’73? Did it really make “only” 275 horsepower in pre-’72 mode with 10.5:1 compression and 2.02-inch intake valves? And what about the 1970 SCCA-inspired 340 Six Pack installed in those special Challenger T/A (2,399 built) and AAR ’Cuda (2,724 built) E-Bodies? Was the claim of its 290 horsepower (5,000 rpm) and 345-lb-ft (3,200 rpm) realistic?

We’ve always wanted to know, and tagged along with Donnie Wood and Steve Chmura of R.A.D. Auto Machine to watch the assembly of a nearly stock-specification 340, and the subsequent dyno test comparison with factory 4-BBL and 6-BBL induction setups. The results was surprising and not surprising at the same time.

On factory claims of 275 horsepower at 5,500 rpm and 340 lb-ft (3,200 rpm), we got 320 horsepower at 5,500 rpm and 368 lb-ft at 3,500 rpm. So it’s clear Mopar understated the facts by 45 horsepower and 28 lb-ft. Then we replaced the manifolds with a set of headers and got 352.1 horsepower and 395.9 lb-ft (at 3,200 rpm)—77.1 horsepower and 55.9 lb-ft higher than published peak claims.

For some in the dyno room, the elevated power readings came as a surprise. Let’s remember, the 340-powered 1968-’69 Dart GTS/Swinger 340 and Barracuda Formula S/’Cuda 340 A-Bodies were initially marketed as budget or “junior” supercars, battling stuff like the SS350 Nova (295 horsepower), W-31 Olds 350 (325 horsepower), Pontiac 350 HO (325 horsepower) and 351 Windsor four-barrel Mustang (290 horsepower). There it would have been great to advertise its real 300-plus horsepower capability against the competition.

But on the other side of the coin, when sold beside 383-powered Road Runners and Super Bees at Plymouth and Dodge dealerships nationwide, the difference between their 335 horsepower rating and the 340’s 320 horsepower may have led to internal competition, and reduced B-Body sales. It’s all water under the bridge at this point, but the 320 horsepower we observed explains a stock 340’s well documented ability to toss any A-Body well into the 14-second zone at the strip. So what about the Six Pack?

Testing RAD’s patience, we asked them to swap intake and exhaust systems on the dyno so we could get more answers. Going back to the factory stock cast-iron exhaust manifolds, a Six Pack replaced the four barrel and we got 356 horsepower at 5,500 rpm and 382 lb-ft at 4,000 rpm. Again, Chrysler’s 1970 claim of 290 horsepower at 5,000 rpm and 345 lb-ft at 3,200 rpm was bettered by 66 horsepower and 37 lb-ft. Finally, we replaced the headers and watched it grow another 20.1 horsepower and 26.9 lb-ft with readings of 376.1 horsepower at 5,500 rpm and 408.9 lb-ft at 4,000 rpm.

So the next time you see a stock 340 or 340 Six Pack, don’t be fooled into thinking it’s a sub-300 horsepower muscle car wannabe. Follow along as we assemble a 340 small-block and put it to the test!

For our rebuild, the stock 4.04-inch bores were enlarged to 4.080 by a 0.040-inch cut on RAD’s Rottler F79A boring machine. Torque plates were bolted to the decks before the Rottler HP6A diamond hone went to work. Sharing the same 3.312-inch stroke with the 273 and 318, the 340’s extra cubes came strictly from its bigger 4.04-inch bores. Bore spacing is 4.46 inches.Like all LA 273, 318, 340, and 360 engines, the displacement is cast into the driver’s side of the block. Ours was cast on 10-1-71 and originally fitted to a 1972 model year car with reduced 8.5:1 compression, electronic ignition, a Carter Thermo-Quad, and a cast crank. Rated power was 240 horsepower at 4,800 rpm with 290 lb-ft at 3,600 rpm. We’ll bring it back to full-pep 1968-’71 status.The cheaper cast-iron cranks used in 1972-’73 340s are safe to 400 horsepower, but RAD had several forged cranks on hand (left) so they made the upgrade. At 55.6 pounds, the forged crank is only 2.6 pounds heavier than the casting. Unlike externally balanced 1972-’78 big-blocks, all 1968-up 273 and 318 cast cranks were internally balanced.With the main saddles aligned on RAD’s Sunnen align hone machine, the Clevite main bearings (PN MS963-P) are installed. The 340 block has thicker bulkheads and wall sections than the 318. Hemi legend Tom Hoover played a key role in developing the A105 340. Read Willem Weertman’s book Chrysler Engines 1922-1998 (SAE Books) for the full story.Some 11.6 pounds lighter than a forged 440 big-block crank, Donnie sets the crank in place. The cap bolts torque to 85 lb-ft in two steps. While 273 and 318 blocks have two core plugs per side, the 340 (and later 360) have three per side.For under $300, forged Eagle cap screw rods (left) share the same 6.123 length and weigh 166 grams less (587 versus 753) than factory items (right). At 893 grams the forged Sealed Power pistons (part No. L2316F) are 23 grams heavier than the stock castings. Refurbishing the stock rods can come close to the cost of superior aftermarket offerings. Times have changed, why go there?Hastings moly-faced rings (part No. 2M694-040) share the same 5/64, 5/65, 3/16 width as stock, and are gapped at 0.018 inch. The Sealed Power pistons feature anti-scuff coating on the skirts and deliver the same 10.5:1 compression as stock. Note the floating pins and clips. The stock 340 pins are also floating type, all other LA pins are press-fit.A mild hydraulic flat-tappet cam from Sealed Power (part No. LS644) with 0.429-/0.442-inch lift, 201-/ 220-degrees duration and 114 degree LSA matches the cam used in 340s mated to the Torqueflite automatic transmission. Four-speed 340s got a slightly hotter cam in 1968 (only) with 0.445-/0.455-inch lift and a few more degrees of overlap. Stock-spec hydraulic lifters are also from Sealed Power (part No. HT211B). The 1970 340 Six Pack used the same Torqueflite-spec cam used in all other 340s that year.Standard-issue windage trays separated the 340 from lesser LA small-blocks until the E58 360 came along in the late seventies. Chrysler claimed it saved 5 to 10 horsepower. Specific main cap fasteners with tapped receiver holes accept the tray hold-down bolts on caps two and four. The stock-spec oil pump (part No. M-72) and 5/8-inch pickup tube and screen (part No. 72-S2) are from Melling.The stock pan was dented and rusty so a new 5-quart unit from Spectra (part No. CRB-09B) went on. Don’t forget, 360 pans are not interchangeable.With the cam thrust plate, Engine Pro double-roller timing set (part No. 3028), fuel pump eccentric, and oil-sling disc in place, a fresh Pioneer die-cast aluminum front cover (part No. 500390) seals the block.After a light surface cut to restore deck flatness, the stock J heads (casting number 3418915-H) were cut for PC valve seals. Bronze wall valve guides were also installed. With the cutting fixture in between, note the stock guide boss (left) and post-surgery boss (right). The outside diameter is reduced to 0.530 inch so PC seals press right on.A “bowl hog” cutter (center) was inserted in the Rottler SGF8 seat machine to lightly dress the throats (left). We argued against this step to keep things stock for testing but were overruled by the engine owner. It’s doubtful this work added more than a few clicks to the results with the mild cam. No other porting or hand blending was performed.Stock-diameter 2.02-/1.60-inch swirl-polished Manley valves replace the burnt stockers. Some 1972 and all 1973 340s were demoted to 1.88-inch intake valves but retained the 1.60 inch exhaust valves.With FelPro Permatorque composite gaskets (part No. 8553PT) in place, the ARP half-inch head bolts were torqued to 100 lb-ft in three steps. The neutral balance crank damper is from Liberty Performance Components (part No. LPB1004ST). The 340 intake port windows measure 2.2 square inches versus 1.60 on the 318 head. The big-port 340 head was adopted for the 360 “family car” engine in 1971 (with smaller 1.88-inch intake valves).We deviated from the stock stamped rocker arms and chosen an internet-sourced roller rocker arm kit. Complete with shafts, fasteners, rocker arms, and even the spacer blocks, the $180 purchase price was amazing. Unfortunately, the undersides of the rocker arms contacted the 1.440-inch diameter valve spring retainers. The pen points to the large gap between roller and valve stem tip during contact.For kicks, Donnie logged onto the AERA online data site (a paid subscription service) and unearthed this service bulletin from January of 1970. Though it isn’t directly applicable to our situation, it confirms the 340’s history of rocker-to-retainer contact. Chrysler’s remedy called for grinding 0.030-inch clearance slots into the stock rocker arms!COMP Cams beehive valve springs (part No. 26986-16) and matching 1.340-inch diameter retainers (part No. 795-16) saved the day. The 10-degree locks were transferred and reused. With 123 pounds on the seat and 284 pounds at 0.500-inch lift, the tapered springs restored the ability to use the roller rocker kit. The pen points to the resulting clearance. Stock 340 valve springs were rated at 96/242 (I/E) pounds and used Hemi-style surge dampers.The roller rocker arm kit rendered the Chrysler’s 7.500-inch 340 push rods too long for the job. Donnie found some 7.266-inch long, 5/16-inch diameter COMP Cams push rods (part No. 7809-16) from a Brand X application. The lifter preload was set at ½ turn beyond zero lash.Auto Metal Direct (AMD) supplied fresh 1970-spec 340 valve covers (part No. EF-3882AR), breather (part No. EF-3881-BTB1), and oil fill cap (part No. EF-3881-CAP). The original parts (in orange) can trap residual grime that can never be fully eradicated. AMD offers year-correct valve covers for all popular small- and big-block applications, not to mention a growing line of excellent, high-precision body panels.Though all 1965-’67 A861 273 four-barrel engines got dual-point distributors, they were only installed on four-speed 340s in 1968-’71. For 1972, Chrysler’s breaker-less electronic ignition was standardized on the 340 (optional in ’71). Our 340 uses a Mopar Performance electronic unit (part No. P3690430, right). Total timing is set at 35 degrees BTDC.The stock dual-plane four-barrel manifold (46.6 pounds) is an excellent piece. The Edelbrock-sourced Six Pack (23.4 pounds) is the only aluminum intake manifold ever offered on the 340. A full two inches taller, the added runner and plenum volume are a huge bonus. Our Six Pack is a 1970 unit; Edelbrock still makes them.There’s no Six Pack setup better than a brand new one, and Holley still sells ’em. Unlike swap meet stuff, no gurus have been inside tinkering, and the optimal factory settings are ready to deliver crisp cruising or neck-snapping, bog-free surges when wanted. The center carb (part No. 0-4792) has 1.50-inch diameter throttle blades and is rated at 350 cfm. The twin 500cfm outboard carbs (part No. 0-4790) are vacuum operated.Lacking a vintage Carter AVS (1968-’70) or Thermo-Quad (1971-’73) we substituted an Edelbrock Performer 1405 for baseline testing. Its 600cfm rating puts it between the 570cfm AVS (1968-’70) and 750cfm TQ (1971-’73), both of which Chrysler claimed made the same 275 horsepower atop the 340.So much for Chrysler’s claims! With the single four-barrel cast-iron factory dual-plane intake and exhaust manifolds, the basically stock 340 revealed the glorious truth with 320.3 horsepower at 5,500 rpm. Torque was 368 lb-ft at 3,500 rpm. There’s no way our minor bowl work, roller-tip rocker arms, and lighter rods added 45.3 horsepower and 28 lb-ft to Chrysler’s claim. The 340 is one potent little animal!OLYMPUS DIGITAL CAMERAWith the simple addition of 1 7/8-inch headers, output jumped to 352.1 horsepower at 5,300 rpm and 395.9 lb-ft of torque. In a 3,200-pound A-Body with 3.91 gears and traction, mid-thirteen second e.t.s are guaranteed.OLYMPUS DIGITAL CAMERAWith the Six Pack installed, we reverted to the factory exhaust manifolds and got 356 horsepower at 5,500 rpm and 382 lb-ft at 4,000 rpm. The trio of Holley 2300-series carburetors were intentionally run straight out of the box. Donnie says, “Resist the urge to fiddle. As long as the linkage and vacuum hose routing is correct, Six Packs are maintenance-free on the street.”No holds barred, the Six Pack with the same 1 7/8-inch headers cranked 376.1 horsepower and 408.9 lb-ft at 4,000 rpm. The air/fuel readings were safe with box-stock calibrations. After testing, the engine was sprayed Street Hemi Orange.

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